A steering apparatus for a vehicle is configured as illustrated in FIG. 6, so that the rotation of a steering wheel 1 can be transmitted to an input shaft 3 of a steering gear unit 2 to push or to pull a pair of right and left tie rods 4 in accordance with the rotation of the input shaft 3, and thus, a steering angle is applied to front wheels. The steering wheel 1 is supported and fixed on the rear end portion of a steering shaft 5. The steering shaft 5, with a cylindrical steering column 6 inserted therethrough along an axial direction, is rotatably supported on the steering column 6 supported on a vehicle body. The front end portion of the steering shaft 5 is connected via a universal joint 7 to the rear end portion of an intermediate shaft 8, and the front end portion of the intermediate shaft 8 is connected via another universal joint 9 to the input shaft 3. An example illustrated in this drawing is an electric power steering apparatus by which the force necessary for operating the steering wheel 1 is reduced with an electric motor 10 used as an auxiliary power source. Herein, a front-rear direction refers to a front-rear direction of a vehicle unless otherwise mentioned.
FIGS. 7 to 9 illustrate an electric power steering apparatus disclosed in Patent Document 1. A steering column 6a is configured by combining an inner column 11 and an outer column 12 so that the total length thereof can be reduced at the time of secondary collision, and is supported on a vehicle body. A steering shaft 5a rotatably supported inside the steering column 6a is configured by combining a lower shaft 13 and an upper shaft 14 so that a torque can be transmitted and that the total length thereof can be reduced at the time of secondary collision. On the rear end portion of the upper shaft 14 protruded from a rear end opening of the outer column 12, a steering wheel 1 (see FIG. 6) is fixed. The front end portion of the inner column 11 is securely joined to a housing 15, and a front part of the lower shaft 13 is inserted into the housing 15. Inside the housing 15, an output shaft 16 is rotatably supported by a pair of ball bearings 17 and 18 on a front side of the lower shaft 13 working as an input shaft. The front end portion of the output shaft 16 protruded from a front end opening of the housing 15 is joined to a universal joint 7 (see FIG. 6). The output shaft 16 and the lower shaft 13 are coupled to each other via a torsion bar 19. Almost all the portion of the torsion bar 19 except its front end portion is disposed on the radially inner side of the lower shaft 13 formed in the shape of a hollow shaft.
The rear end portion of the output shaft 16 is provided with a cylindrical portion 20. On the inner peripheral surface of the cylindrical portion 20, a female stopper 21 having a concave-convex profile along the circumferential direction is provided. The female stopper 21 includes, on the inner peripheral surface of the cylindrical portion 20, a plurality of female teeth 22 and a plurality of female grooves 23. The plural female teeth 22 and the plural female grooves 23 extend in the axial direction and are arranged alternately along the circumferential direction at equal pitches. A front end portion of the outer peripheral surface of the lower shaft 13 has a male stopper 24 having a concave-convex profile along the circumferential direction. The male stopper 24 has, on the front end portion of the outer peripheral surface of the lower shaft 13, a plurality of male teeth 25 and a plurality of male grooves 26. The male teeth 25 and the male grooves 26 extend in the axial direction and are arranged alternately along the circumferential direction at equal pitches. The number of the male teeth 25 (the male grooves 26) and the number of female grooves 23 (the female teeth 22) are the same. The male stopper 24 and the female stopper 21 are engaged with each other in a concave-convex engaging manner (like a loose spline-engagement) so as to be relatively rotatable within a given angle range (e.g., ±5 degrees from a neutral state where the torsion bar 19 is not twisted). Thus, the torsion bar 19 is prevented from excessively twisting.
The lower shaft 13 is made of steel, that is, a magnetic metal. An outer peripheral surface of a portion of the lower shaft 13 near its front end and adjacent to the rear side of the male stopper 24 with respect to the axial direction is provided with a torque detection concave-convex section 27 having a concave-convex profile along the circumferential direction and having a circumscribed circle with a larger diameter than the male stopper 24. This torque detection concave-convex section 27 is configured by providing, on the circumference of the portion of the lower shaft 13 near its front end, a plurality of detection grooves 28 respectively extending in the axial direction at equal intervals in the circumferential direction. In the structure illustrated in this drawing, the diameter of a groove bottom circle of the torque detection concave-convex section 27 and the diameter of a groove bottom circle of the male stopper 24 are the same. The number of detection grooves 28 and the number of male grooves 26 are the same, and the male grooves 26 and the detection grooves 28 are provided continuously (in a state having the same phase along the circumferential direction) in the axial direction.
On the radially outer side of the torque detection concave-convex section 27, a torque detection sleeve 29 in a cylindrical shape made of a nonmagnetic metal having conductivity such as an aluminum alloy is provided. The base end portion of the torque detection sleeve 29 is fitted outside and fixed on the cylindrical portion 20. In a portion of the torque detection sleeve 29 positioned on the radially outer side of the torque detection concave-convex section 27, a plurality of window holes 30 are provided. On the radially outer side of the torque detection concave-convex section 27 and the torque detection sleeve 29, a torque detection coil unit 31 securely fitted in the housing 15 is provided.
A worm wheel 32 is fitted outside and fixed on a portion of the output shaft 16 near its rear end. The worm wheel 32 is engaged with a worm 33 rotatably supported in the housing 15. In the housing 15, an electric motor 10 (see FIG. 6) is supported and fixed. The output shaft of the electric motor 10 is joined to the base end portion of the worm 33 to be capable of transmitting a torque.
In the electric power steering apparatus configured as described above, when a torque corresponding to a steering force is applied to the steering shaft 5a by a driver operating the steering wheel 1, the torsion bar 19 is elastically twisted (within the given angle range) in accordance with the direction and the size of this torque. This elastic twist changes the positional relationship in the circumferential direction between the torque detection concave-convex section 27 and the torque detection sleeve 29, and hence an impedance change is caused in a coil of the torque detection coil unit 31. On the basis of this impedance change, the direction and the size of the torque can be detected. The electric motor generates auxiliary power in accordance with the detection result of the torque. The auxiliary power is increased by a worm speed reducer 34 configured by engaging the worm 33 and the worm wheel 32 with each other, and then applied to the output shaft 16. As a result, the force required for the driver to operate the steering wheel 1 is reduced.
On the other hand, when the amount of twist of the torsion bar 19 reaches an upper limit value on one side or the other side of the given angle range because of a large torque input from the steering wheel 1 to the steering shaft 5a, the teeth 22, 25 of the female stopper 21 and the male stopper 24 are engaged with each other in the circumferential direction. Then, on the basis of this engagement, a part of the toque is directly transmitted from the lower shaft 13 to the output shaft 16.
In the electric power steering apparatus described above, the thickness in the radial direction of a portion of the cylindrical portion 20 disposed on the radially outer side of the groove bottom circle of the female stopper 21, and the height in the radial direction of the respective female teeth 22 are set to dimensions for sufficiently securing the mechanical strength of the cylindrical portion 20, namely, set to dimensions not causing plastic torsional deformation of the cylindrical portion 20 in the torque transmission performed based on the engagement between the teeth 22, 25 of the female stopper 21 and the male stopper 24 described above.
In such an electric power steering apparatus, if the diameter of the circumscribed circle of the torque detection concave-convex section 27 is to be reduced for reducing the diameters of the respective components for the torque detection and portions around them, it is necessary to also reduce the inner diameter dimension of the torque detection sleeve 29 in accordance with the diameter of the circumscribed circle of the torque detection concave-convex section 27. Besides, in accordance with the inner diameter dimension of the torque detection sleeve 29, it is necessary to reduce the outer diameter dimension of the cylindrical portion 20 onto which the base end portion of the torque detection sleeve 29 is inserted and fixed. In this case, in order to retain the mechanical strength of the cylindrical portion 20, if the outer diameter dimension of the cylindrical portion 20 is to be reduced without changing the thickness in the radial direction of the portion of the cylindrical portion 20 disposed on the radially outer side of the groove bottom circle of the female stopper 21 and the height in the radial direction of the respective female teeth 22, it is necessary to reduce also the diameters of the circumscribed circle and the groove bottom circle of the male stopper 24 in accordance with the diameter dimension of the cylindrical portion 20 for avoiding interference between the tooth tops and the groove bottoms of the female stopper 21 and the male stopper 24.
In the configuration employed by the above-described conventional structure, however, the diameter of the groove bottom circle of the male stopper 24 and the diameter of the groove bottom circle of the torque detection concave-convex section 27 are the same. Therefore, if the diameter of the groove bottom circle of the male stopper 24 is reduced as described above, the diameter of the groove bottom circle of the torque detection concave-convex section 27 is also accordingly reduced. In accordance with this diameter reduction, in the portion of the lower shaft 13 near its front end, the thickness in the radial direction of a portion on the radially inner side of the groove bottom circle of the torque detection concave-convex section 27 is reduced. As a result, in the torque transmission performed based on the engagement between the teeth 22, 25 of the female stopper 21 and the male stopper 24, a torque that can be transmitted without causing the plastic torsional deformation of the torque detection concave-convex section 27 is small.